Magnetic focusing of x-ray tubes and system for operating



March 19, 1968 L. G. PARRATT ET AL MAGNETIC FOCUSING OF 'X-RAY TUBES ANDSYSTEM FOR OPERATING Filed Feb. 21, '1946 2 Sheets-Sheet 1 [N VEN T0115Lyman G. Pan-rail Trev-or R. Cugkendall Kenneth I Grez'sen L. TaylorFinlorgson Frederick Reines March 19, 1968 PARRATT ET AL 3,374,355

MAGNETIC FOCUSING 0F X-RAY TUBES AND SYSTEM FOR OPERATING Filed Feb. 21,1946 2 Sheets-$heet 2 Oscilloscope Dejlecaion Time in Millisecozzd'sFig. 4

' INVENTORS.

Lyman 6. Parrafi Trevor R. Cu Irendall Kennah I. reisen L. TaylorFinlqgson Frederick Reines BY Maw.

United States Patent*() 3,374,355 1 MAGNETIC FOCUSING F X-RAY TUBES ANDSYSTEM FOR OPERATING Lyman G. Parratt, Trevor R. Cuykendall, Kenneth I.Greisen, L. Taylor Finlayson and Frederick Reines, Santa Fe, N. Mex.,assignors to the United States of America as represented by the UnitedStates Atomic Energy Commission i Filed Feb. 21, 1946, Ser. No. 649,413,1 Claim. (Cl. 250-9.9)

This invention relates generally to an improved method and to apparatusfor controlling the initiation of X-rays.

A primary object of the invention resides in the provi sion of animproved method f0. the reduction of the focal spot size within an X-raytube which is adapted to deliver an X-ray burst of extremely short timeduration.

A further object is to improve the operation of an X-ray tube throughimproved control of the electron stream intermediate the cathode and theanode in a manner'ultimately to decrease the effective target area.

A still further object of the invention is to decrease the focal spotsize within an X-ray tube pulsed in the time order of milliseconds ormicroseconds, including fractional portions thereof.

Another object of the invention is to provide an improved apparatus foroperating an X-ray tube during ex.- tremely short time durations whileconfining electron flow therein intermediate the cathode and target tosubstantially linear paths.

A still further object of the invention is to provide a method andapparatus for confining electron flow between the cathode and anode of apulsed X-ray tube to convergent paths for the reduction of the focalspot size.

Another important object of the invention resides in providing an X-raytube adapted to be operated in response to a pulsed interelectrodecurrentwith a magnetic focusing means adapted to be operated in responseto a pulsed current including electronic control for synchronizingelectron beam flow within the period of maximum magnetic focusing flux.

Other objects and advantages of the present invention Will becomeapparent to persons skilled in the art upon examination of the followingdescription, the drawings and the appended claims.

In the drawings:

FIGURE 1 is a fragmentary view in longitudinal section illustrating amethod for confining electron flow intermediate the tube electrodes tosubstantially linear paths.

FIGURE 2 illustrates in fragmentary longitudinal section a method ofconfining the electron flow to convergent paths.

FIGURE 3 illustrates in block diagram a circuit arrangement of apparatussuitable for carrying out the present invention.

7 FIGURE 4 is a graph showing oscilloscope deflection of essentially themagnitude of flux density the focusing plotted against time.

While magnetic focusing of an electron beam is old per se, for examplein the cathode ray oscilloscope art, and has also been previouslyemployed in the X-ray art, the present invention relates to an improvedmethod and t0 apparatus for applying magnetic focusing to' an electronbeam pulsed in the order of milliseconds or microseconds includingfractional portions thereof, in a manner to provide an X-ray pulse oftime duration and intensity long sought but not heretofore obtained inthe X-ray art.

The utilization of X-rays to permit photographing of objects, especiallywhile under very rapid motion, for example a bullet in flight orfragments of a bursting shell, or an object under collapse orcompression by implosive forces, requires in addition to a shortexposure time in- Patented Mar. 19, 1968 terval of a microsecond orless, a small focal spot, i.e., a small area of X-ray emission from thetarget of the X-ray tube. The present invention permits substantialreduction of the size of the focal spot with the result thatditficulties involving presence of penumbra in the photograph areminimized. While various methods have been employed in the X-ray art toobtain reduction in effective target area, the focal spots of all knownX-ray tubes designed to operate at sufficiently high speeds and highpotential for the above use are too large to produce a clearly definedumbra. All known X-ray tubes have an electrostatic field, including thefield due to space charge in the electron stream between the cathode andanode, which results, during operation, in a divergent rather than aparallel or convergent electron stream between the cathode and the anodeor target. Many unsuccessful attempts have previously been made tocontrol the shape of the electron stream in a manner ultimately todecrease the effective target area.

By introducing a magnetic field having a suitable configuration betweenthe cathode and the target of an X-ray tube the paths defined byelectrons in passing from cathode to target may be controlled so as toelfect reduction of the focal spot diameter. Such a magnetic field canbe established by disposing a cylindrical coil externally of the tubeand concentric with the axis thereof, as illustrated in FIGURE 1, thecoil being designated 10. It is assumed that coil 10 is connected to asuitable source of energizing potential, not shown, and that theillustrated anode 11 and cathode 12 enclosed within a suitable evacuatedenvelope'13 are connected to suitable energizing potentials in a mannerwell known in the art. The magnetic lines of force established by coil10 within the area intermediate cathode l1 and anode or target 12, areshown by the dotted lines 15, which are substantially parallel withinthe axis ofthe tube through that area. If the magnetic field issufiiciently strong, electrons emanating from cathode 11 will followlines of force 15 from cathode 11 to target 12 along paths which may beconsidered linear and parallel, although individual electrons willtranscribe helical and/or spiral paths about the lines of force. Sincein the absence of a magnetic field such as provided by coil 10 theelectrostatic lines intermediate cathode 11 and target 12 diverge fromcathode 11 to target 12, employment of such a magnetic field cancels theundesirable effect obtained by the electrons following divergentelectrostatic lines of force intermediate the electrodes, hence a focalspot of substantially reduced size is obtained.

Referring to FIGURE 2 an alternate method of obtaining a focal spot ofdecreased size is illustrated. Coil 110, concentrically enclosing acylindrical portion of tube encelope 113 functions in the same manner asdoes coil 10 abovedescribed. While cathode 111 may be similar to oridentical with cathode. 11 of the apparatus of FIG- URE 1, the anode ortarget 112- is of tubular construction to permit the insertion of a rod20' of magnetic material, for example soft iron, the function of rod20'being to provide decreased reluctance throughout an area, for examplethe central area of the target 112 to effect convergence of the magneticlines of force as illustrated by the dotted lines 115. Since thefunction of rod 20 is solely to concentrate magnetic flux linesthroughout an area, this rod need not be at the potential of the target112, however since itis contiguous to the inner target surface it may bedesirable to maintain rod 20 at or near the target potential topreventfarc-overs. While the method of target size reductionillustrated'in connection with the apparatus of FIGURE 2 has been foundsatisfactory for the generation of X-ray pulses in the time order ofmicroseconds, it has equal utility for use in connection with thegeneration of continuous X-rays. The circuit layout of equipmentparticularly adapted to carry out the present invention is shown inFIGURE 3. Since the individual circuits or elements are well known inthe electronics art, they are shown in block diagram. Persons skilled inthe art will encounter no difficulty in constructing the apparatus shownfrom known circuit components. The magnetic focusing coil 10,concentrically enclosing an X-ray tube as above described, is connectedthrough an electrically operable switch and through resistors 26, 27,28, and 29 to a source of energizing potential, not shown, which may bein the order of several kilovolts of direct potential. A suitable chargestoring capacitor 31 is associated in shunt with the potential source ina manner to be discharged through coil 10 upon the closing of switch 25.The value of capacitor 31 is such that the discharge thereof initiateswithin coil 10- a strong surge current resulting in the rapidmagnetization thereof as shown by the flux density curve a, FIGURE 4, inwhich the flux density is plotted against rise and fall times inmilliseconds.

Concurrent with the establishment of the surge current within coil 10, arapid potential change occurs across resistors 27 and 28, a portion ofwhich is utilized to control timing of the electron flow between thecathode and the anode of an associated X-ray tube, not shown, in thefollowing manner.

Resistor 28 comprises a potentiometer, the variable tap 32 of whichleads through conductor 33 to the input of low pass filter 34 having itsoutput connected through conductor 35 to the input of a thyratrontriggering circuit 38 which includes a variable bias control indicatedat 39. The output of thyratron circuit 38 leads through conductor 41 toan X-ray triggering circuit 43 which associates through conductor 45with a surge generator 47. A rapid potential change occurring acrossthat portion of potentiometer 28 intermediate the grounded end thereofand the variable tap 32 is employed to initiate a proper signal forswinging thyratron triggering circuit 38 from a normally nonconductingstate to a conducting state. Circuit 38 includes means, not shown, forthe initiation of an output signal in response to the above-mentionedtriggering signal, the output signal of which acts through X-raytriggering circuit 43 to fire the surge generator circuit 47 which isconnected in a known manner, not shown, between the cathode and theanode of the X-ray tube to be controlled. The surge generator circuit 47is preferably of the type wherein discharge of a condenser bank resultsin an output surge in the order of microseconds.

The proper time delay relation between the application of the focusingcoil energizing current by closure of switch 25 and application of theinterelectrode surge by firing of surge generator circuit 47 is effectedin the following manner. Thyratron circuit 38 is adjusted to a value ofthreshold bias sufiicient to maintain a nonconducting state thereinuntil the flux density rise within coil 10 approaches a maximum. Sincethe potential change be tween that portion of potentiometer 28 connectedbetween ground and variable tap 32 is proportional to the change of fluxdensity within coil 10, proper adjustment of potentiometer 28 in respectto a suitable bias adjustment of circuit 38 may be obtained to give thedesired effect. Referring to FIGURE 4, dotted line b may illustrate thedesired value of flux density in respect to a maximum value thereofwhich is required to fire circuit 38 and in this respect the value offlux density also represents the RI drop across the controlling portionof potentiometer 28.

Assuming that no delay exists between the firing of circuit 38 andfiring of surge generator 47, the surge generator would of course firesimultaneously, but since circuits 38, 43, and 47 all include someinherent delay, this delay has been utilized to effect a desired lagbetween the firing of circuit 38 and firing of circuit 47. If theinherent delay of circuits of this type is found to be insufficient forthe purpose desired, the delay of any one of these circuits may beincreased in a manner well known in the art.

Referring again to FIGURE 4, dotted line 0 represents the point at whichmaximum flux is established within coil 10, at which instant surgegenerator circuit 47 fires to produce the peak current illustrated at e.

Since it is desirable to determine the adjustment of the circuit toinsure proper timing of the sur e generator output in respect to themagnetizing coil flux, cathode ray oscillograph apparatus may beassociated therewith in the following manner. Since the circuit abovedescribed is of the type having one side grounded, a conduit 49 leadingfrom the coil connecting ends of resistors 27 and 28 through a capacitor51 to the input of a suitable amplifier circuit 52 will provide an inputsignal to amplifier 52 proportional to the potential change occurringacross resistors 27 and 28. The output of amplifier 52 connects by meansof conduits 54 and 56 to the vertical plates 58 and 60 of a cathode rayoscillograph tube 62. A second conduit 63 originating at the same pointleads through a thyratron triggering circuit 65 into an amplifier andintensity control circuit 69 the output of which is connected throughconduits 70 and 72 to the horizontal plates 74 and 76 of tube 62.Intensity control within circuit 69 is illustrated by arrow 78. Thefunction of thyraton circuit 65 is to initiate a single sweep acrosstube 62 in response to the initiation of current flow within themagnetizing coil circuit which is of the type adapted to preventsubsequent sweeps and well known in the art. The function of capacitor51 is of course to block direct current from entering amplifier 52.Since it is desirable to indicate both the rise and fall of flux densityand surge generator discharge as vertical components, the surgegenerator discharge may be imposed upon the flux density curve byinterposing a unidirectional buffer circuit 80, a blocking capacitor 81being provided between discharge circuit 38 and amplifier circuit 52,the output of circuit 80 and the input of circuit 52. With such anarrangement the output signals from thyratron circuit 38 rather than thepulse discharge output of circuit 47 are employed to provide a verticalpip as shown in e, FIGURE 4. This arrangement is satisfactory, providedcircuit 38 contains most of the inherent delay above mentioned and noappreciable delay exists in either circuit 43 or 47. If however circuit43 also has inherent delay the input of circuit 80 may be connected tothe output of circuit 43 instead of the other output of circuit 38.Since the oscillograph apparatus merely includes conventional equipmentand circuit arrangement further detail is avoided and the invention isnot to be limited in any manner to the inclusion of any specificindicating device.

While the above apparatus includes a preferred form of our invention,other adaptations and modifications thereof will occur to personsskilled in the art without departure from the spirit and scope thereof.

We claim:

1. A system for operating an X-ray generator comprising in combinationan X-ray tube having an axially spaced anode and electron emissivecathode, a coil surrounding said tube for producing a magnetic field inthe direction of said axis; means for rapidly producing a pulsed intensemagnetic flux comprising a source of surge current, and means forconnecting said surge current to said coil, means responsive to aselected predetermined magnitude of said surge current for applying asource of surge voltage to said anode and said cathode, means fordeveloping a potential having an amplitude variable in time inaccordance with said flux, a cathode ray oscilloscope, means forapplying said potential to one pair of deflecting plates of saidoscillator, means for applying a sweep voltage to the other of saiddeflecting plates simultaneously with the application of said surgecurrent to said coil, and means for applying the surge 5 voltagedischarge also to said first named defiection plates, whereby saidpredetermined magnitude may be selected so that said'surge voltage isapplied when the magnetic flux produced by said coil is maximum.

References Cited UNITED STATES PATENTS 1946 288 2/1934 Kearsley 250 99JAMES L. BREWRINK, orro W. STRACHAN,

Examiners. 2,392,330 1/1946 Varian 251 -93 X 2,394,071 2/1946 Westendorf2s0-93 X 10 LINSCOII Amid"! Emml'ler- 6 FOREIGN PATENTS 444,788 3 1936British. 292,317 6/1916 German. 704,121 2/1941 German.

RALPH G. NILSON, Primary Examiner.

1. A SYSTEM FOR OPERATING AN X-RAY GENERATOR COMPRISING IN COMBINATIONAN X-RAY TUBE HAVING AN AXIALLY SPACED ANODE AND ELECTRON EMISSIVECATHODE, A COIL SURROUNDING SAID TUBE FOR PRODUCING A MAGNETIC FIELD INTHE DIRECTION OF SAID AXIS; MEANS FOR RAPIDLY PRODUCING A PULSED INTENSEMAGNETIC FLUX COMPRISING A SOURCE OF SURGE CURRENT, AND MEANS FORCONNECTING SAID SURGE CURRENT TO SAID COIL, MEANS RESPONSIVE TO ASELECTED PREDTEREMINED MAGNITUDE OF SAID SURGE CURRENT FOR APPLYING ASOURCE OF SURGE VOLTAGE TO SAID ANODE AND SAID CATHODE, MEANS FORDEVELOPING A POTENTIAL HAVING AN AMPLITUDE VARIABLE IN TIME INACCORDANCE WITH SAID FLUX, A CATHODE RAY OSCILLOSCOPE, MEANS FORAPPLYING SAID POTENTIAL TO ONE PAIR OF DEFLECTING PLATES OF SAIDOSCILLATOR, MEANS FOR APPLYING A SWEEP VOLTAGE TO THE OTHER OF SAIDDEFLECTING PLATES SIMULTANEOUSLY WITH THE APPLICATION OF SAID SURGECURRENT TO SAID COIL, AND MEANS FOR APPLYING THE SURGE VOLTAGE DISCHARGEALSO TO SAID FIRST NAMED DEFLECTION PLATES, WHEREBY SAID PREDETERMINEDMAGNITUDE MAY BE SELECTED SO THAT SAID SURGE VOLTAGE IS APPLIED WHEN THEMAGNETIC FLUX PRODUCED BY SAID COIL IS MAXIMUM.